Abstract. Optical polarimetry observations on La Palma, Canary Islands, during a
Saharan dust episode show dichroic extinction indicating the presence of
vertically aligned particles in the atmosphere. Modelling of the extinction
together with particle orientation indicates that the alignment could have
been due to an electric field of the order of 2 kV/m. Two alternative
mechanisms for the origin of the field are examined: the effect of reduced
atmospheric conductivity and charging of the dust layer, the latter effect
being a more likely candidate. It is concluded that partial alignment may be
a common feature of Saharan dust layers. The modelling indicates that the
alignment can significantly alter dust optical depth. This "Venetian blind
effect" may have decreased optical thickness in the vertical direction by
as much as 10% for the case reported here. It is also possible that the
alignment and the electric field modify dust transport.